This invention relates generally to vacuum extraction systems for recovering liquid hydrocarbons from ground water, and more particularly concerns a vacuum extraction system which removes all of the liquid hydrocarbons from a well bore and which provides control of the flow of liquid from each well within a multiple well field to assure equilibrium among the wells being serviced by the vacuum extraction system.
At petroleum handling facilities such as refineries, storage facilities, terminal facilities, and gasoline stations, spillage of liquid hydrocarbons can result in the contamination of ground water in the immediate vicinity. The problem of ground water contamination can occur as a result of slow leakage over time or a more catastrophic spillage event. In either case, the liquid hydrocarbons seep through the ground to the level of the ground water table. Because liquid hydrocarbons have specific gravities that are less than water and are generally immiscible with water, they form a layer on top of the ground water table.
Conventionally, in order to remove the contaminating liquid hydrocarbons from the ground water, it has been necessary to drill a number of bore holes or drive a number of well points in the area where the contamination exists and then extract large quantities of ground water to create a cone of depression in the ground water table adjacent each of the well bores or well points. Gravity forces the liquid hydrocarbons to flow toward the center of the cones of depression, and the liquid hydrocarbons collect there within each well bore or well point. In some areas where the ground water table level is shallow, it may be possible to adopt a vacuum extraction system for servicing the wells. A vacuum extraction system is limited in the depth from which it can draw water and liquid hydrocarbons to the surface. As a practical matter, with losses in the system, vacuum extraction systems are generally limited to about 20 to 25 feet of depth.
A conventional vacuum extraction system consists of well points driven into the ground to a predetermined depth. The outlet port of each well point is then connected through a restrictor to a vacuum manifold. As the extraction progressed, the restrictors are adjusted to assure that none of the well points in the system is pulling air into the manifold. The adjustment is typically done by means of a stethoscope pressed to the outlet port adjacent the restrictor by which a technician could listen for the sound of air passing through the outlet port of each individual well point. Such prior vacuum extraction systems are incapable of removing all of the accumulated liquid hydrocarbons once the hydrocarbon layer becomes thin on top of the water table. As the cone of depression around each well point sinks to the level of the intake screen of the well point, and the liquid hydrocarbons begins flowing into the intake, there is a very short distance (the thickness of the liquid hydrocarbon layer) between the entry of the liquid hydrocarbons into the well point and the entry of air at the interface between the liquid hydrocarbons and the air. Consequently, just at the point where the well point begins drawing liquid hydrocarbons, any small drop in the liquid level causes the well point to begin drawing air. When the well point begins drawing air, the technician closes down the restrictor to limit the flow thereby allowing the cone of depression adjacent the well point to rise so that the well point extracts water and not the liquid hydrocarbons as intended. Moreover, the presence of the restrictor at the outlet of the well point adjacent the vacuum manifold assures that any liquid hydrocarbons that are extracted by the well point are thoroughly emulsified as they pass through the restrictor thus rendering separation and recovery expensive if not nearly impossible. For well points that are driven too deep, those particular well points may draw water and no liquid hydrocarbons throughout the entire extraction process. Also, there is no way of raising or lowering the well point to compensate for the depth of the water, liquid hydrocarbon, and air interface even if the depth of that interface depth could be determined through use of the well point, which it cannot.